An optical modulation mechanism based on dynamically shifting the photonic potential barrier of a photonic crystal waveguide is presented. The modulation mechanism is modeled by the one-dimensional quantum tunneling effect using the Schrödinger equation. The calculation results show that the modulation efficiency is 200 times higher than that of the conventional Mach-Zehnder modulator. Based on this innovative concept, an engineering design of an ultracompact silicon photonic crystal waveguide modulator with 10 microm x 5 microm footprint is presented.